The present invention relates to a sliding structure, and more particularly to a sliding structure such as a compressor or the like used in room air conditioners and refrigerations, and to a sliding structure of which sliding section is formed of a material combination that has wear resistance and high economic efficiency suitable for achieving high performance and high reliability.
There are a wide variety of compressors for use in room air conditioners and refrigerators, including a rotary type, a reciprocating type, a scroll type and a screw type. Above all, the typical one is a rotary compressor.
The rotary compressor comprises a crankshaft supported by upper and lower bearings jointly constituting a journal bearing, a roller eccentrically rotated by the crankshaft, a cylinder housing the roller therein, and a vane freely slidably disposed in a vane groove formed in the cylinder, the distal end of the vane being held in slidable contact with the outer periphery of the roller. The sliding members constituting the rotary compressor operate to compress Freon gas under lubrication by a refrigerating machine oil that is dissolving Freon gas. Those sliding members require appropriate lubricated conditions and wear resistance. Thus, it has been general in the conventional rotary compressor to employ iron base sliding members; specifically, the upper and lower bearings are formed of flaky graphite cast iron or an iron base sintered material, and the crankshaft is formed of eutectic graphite cast iron, or spheroidal graphite cast iron or flaky graphite cast iron.
Recently, however, there is a tendency toward a small-sized, high-output rotary compressor of rotational speed control type. This accompanies such fears that, under operation resorting to a lubrication oil film of a refrigerating machine oil of low viscosity diluted with Freon (trademark), a boundary lubrication region may be caused along with metal-to-metal contact occurring due to insufficient oil film existence during high-load, low-speed operation or during quick start operation, and that insufficient oil film existence may be accelerated due to an increase in the coefficient of friction and the amount of wear, as well as intrusion of worn material dust and fine foreign matters, thereby impairing reliability of the compressor over a long period of time.
In consideration of such fears, several improvements explained below have been proposed in the prior art to increase wear resistance of respective sliding members. However, each improvement has respective advantages and disadvantages in view of the other. That is, there is not yet found an optimum material combination that meets requirements of both wear resistance and productivity.
For example, Japanese Patent Publication No. 55-4958 discloses a rotary compressor using a combination of iron base sintered alloys such that a cylinder of cast iron and both or either one of a roller and a vane are subjected to soft nitriding. But, the soft nitriding of iron base sintered alloys in the form of porous material develops a so large configurative deformation that vacancies, pores and nitrides cause the notch effect, which results in a problem of strength.
Japanese Patent Laid-Open No. 60-73082 discloses such a combination that the inner surface of a cylinder is formed of an iron base sintered alloy containing iron oxide of 10-40 volume %, while a roller and a vane are formed of an iron base sintered alloy in which metal carbide and metal oxide are dispersed in the matrix produced by tempering martensite, and nitrogen is contained in the matrix in solid-solution state. But, this materials combination is significantly inferior in strength to a conventional material produced by a melting method, when used for a vane of a compressor of small-sized, high-output high-functional, high-performance type.
Japanese Patent Laid-Open No. 62-13784 discloses a crankshaft which is immersed in a salt bath mainly including alkali metal cyanate to form a porous layer of iron nitride containing iron sulfide and an underlying alloy layer of iron nitride disposed thereunder. But, this is disadvantageous as follows. The salt bath components of high toxicity enter hollow portions and oil bores of the crankshaft and are apt to remain there as washing residue. The step of making resultant waste liquid harmless is required, which remarkably degrades productivity and economical efficiency. Furthermore, in the case of a member which needs control of high dimensional accuracy, correction machining is required to ensure the dimension after treatment.
In addition, Japanese Patent Laid-Open No. 61-36166 discloses a sliding member of which sliding face is formed of ceramic. Specifically, the sliding member is made of a material containing alumina as a main constituent and zirconia of 5-50 weight % or inorganic fibers of 5-50 weight % mixed therewith. In the ceramic of Al.sub.2 O.sub.3 mixed with zirconia, however, nonstabilized zirconia converts from the tetragonal system to the monoclinic system due to transformation and expansion, thereby causing a great number of fine cracks inside the alumina sintered material. This results in a problem of strength and hence makes it difficult to use the alumina sintered material as a sliding member for high load.
In the above-mentioned prior art it is not taken into consideration to obtain sliding members of high strength used for driving a small-sized, high output, high-performance compressor, while fully satisfying all of the following requirement: sufficient mechanical strength; sufficient oil-retainability, compatibility and wear resistance under the boundary lubrication conditions caused by a refrigerating machine oil of low viscosity diluted with Freon; improving the problem of washing the salt bath components adhered; and enhancing production efficiency of a complex post-steps such as machining to final size. Thus, these points are still left unsolved. Further, a compressor using Freon has had many problems in increasing the strength of a material of sliding members to cope with tendency toward smaller size and higher performance, enhancing the mechanical performance such as mechanical loss and volume efficiency of the compressor, improving reliability of operation over a long period of time, and suppressing the production cost.
Although the problems have been explained especially for the rotary compressor as a typical one of various compressors, the similar problems are also encountered in other compressors of reciprocating type, scroll type and screw type, for example.